Resonance Raman Optical Activity Spectra Research Articles

Why Does One Measure Resonance Raman Optical Activity? A Unique Case of Measurements under Strong Resonance versus Far-from-Resonance Conditions.

Raman optical activity (ROA) spectroscopy exhibits significant potential in the study of (bio)molecules as it encodes information on their molecular structure, chirality, and conformations. Furthermore, the method reveals details on excited electronic states when applied under resonance conditions. Here, we present a combined study of the far from resonance (FFR)-ROA and resonance ROA (RROA) of a single relatively small molecular system. Notably, this study is the first to employ the density functional theory (DFT) analysis of both FFR-ROA and RROA spectra. This is illustrated for cobalamin derivatives using near-infrared and visible light excitation. Although the commonly observed monosignate RROA spectra lose additional information visible in bisignate nonresonance ROA spectra, the RROA technique acts as a complement to nonresonance ROA spectroscopy. In particular, the combination of these methods integrated with DFT calculations can reveal a complete spectral picture of the structural and conformational differences between tested compounds.

Molecular Properties of 3d and 4f Coordination Compounds Deciphered by Raman Optical Activity Spectroscopy.

Molecular properties of coordination compounds can be efficiently studied by vibrational spectroscopy. The scope of Raman spectroscopy has been greatly enhanced by the introduction of Raman optical activity (ROA) sensitive to chirality. The present review describes some of its recent applications to study the coordination compounds. 3d and 4f metal complexes often absorb the excitation light, or exhibit luminescence. Therefore, effects caused in ROA spectra by electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) must be taken into consideration.In 3d metal complexes ECD and circularly-polarized Raman scattering compete with the resonance ROA (RROA) signal. Pure RROA spectrum can thus be obtained by subtracting the so-called ECD-Raman component. CPL is frequently encountered in 4f systems. While it can mask the ROA spectra, it is useful to study molecular structure. These electronic effects can be reduced by using near-infrared excitation although vibrational ROA signal is much weaker compared to the usual green laser excitation scenario. The ROA methodology is thus complex, but capable of providing unique information about the molecules of interests and their interaction with light.

Recognition of the True and False Resonance Raman Optical Activity.

Resonance Raman optical activity (RROA) possesses all aspects of a sensitive tool for molecular detection, but its measurement remains challenging. We demonstrate that reliable recording of RROA of chiral colorful compounds is possible, but only after considering the effect of the electronic circular dichroism (ECD) on the ROA spectra induced by the dissolved chiral compound. We show RROA for a number of model vitamin B12 derivatives that are chemically similar but exhibit distinctively different spectroscopic behavior. The ECD/ROA effect is proportional to the concentration and dependent on the optical pathlength of the light propagating through the sample. It can severely alter relative band intensities and signs in the natural RROA spectra. The spectra analyses are supported by computational modeling based on density functional theory. Neglecting the ECD effect during ROA measurement can lead to misinterpretation of the recorded spectra and erroneous conclusions about the molecular structure.

Resonance Raman Optical Activity Spectroscopy in Probing Structural Changes Invisible to Circular Dichroism Spectroscopy: A Study on Truncated Vitamin B12 Derivatives.

This work demonstrates resonance Raman optical activity (RROA) spectra of three truncated vitamin B12 derivatives modified within the nucleotide loop. Since truncated cobalamins possess sufficiently high rotational strength in the range of ROA excitation (532 nm), it was possible to record their spectra in the resonance condition. They showed several distinct spectral features allowing for the distinguishing of studied compounds, in contrast to other methods, i.e., UV-Vis absorption, electronic circular dichroism, and resonance Raman spectroscopy. The improved capacity of the RROA method is based here on the excitation of molecules via more than two electronic states, giving rise to the bisignate RROA spectrum, significantly distinct from a parent Raman spectrum. This observation is an important step in the dissemination of using RROA spectroscopy in studying the complex structure of corrinoids which may prove crucial for a better understanding of their biological role.

Resonance Raman Optical Activity Shows Unusual Structural Sensitivity for Systems in Resonance with Multiple Excited States: Vitamin B12 Case.

In this work, cobalamins with different upper axial substituents and a cobalamin derivative with a ring modification were studied using chiroptical spectroscopies, in particular resonance Raman optical activity (RROA), to shed light on the influence of structural modifications on RROA spectra in these strongly chiral systems in resonance with multiple excited states at 532 nm excitation. We have demonstrated that for these unique systems RROA possesses augmented structural specificity, surpassing resonance Raman spectroscopy and enabling at the same time measurement of cobalamins at fairy low concentrations of ∼10–5 mol dm–3. The enhanced structural specificity of RROA is a result of bisignate spectra due to resonance via more than one electronic state. The observation of increased structural capability of RROA for cobalamins opens a new perspective for studying chiral properties of other biological systems incorporating d-metal ions.

Resonance Effects in the Raman Optical Activity Spectrum of [Rh(en)3]3.

Raman optical activity spectra of Λ-tris(ethylenediamine)-rhodium(III) {[Rh(en)3]3+} have been calculated at 16 on-, near-, and off-resonant wavelengths between 290 and 800 nm. The resulting spectra are analyzed in detail with a focus on the observed resonance effects. Because several electronically excited states are involved, the spectra are never monosignate, as is often observed in resonance Raman optical activity spectra. Most normal modes are enhanced through these resonance effects, but in several cases, de-enhancement effects are found. The molecular origins of the Raman optical activity intensity for selected normal modes are established by means of group coupling matrices. In general, this methodology allows one to produce an intuitive explanation for the intensity behavior of a given normal mode. However, due to the complex electronic structure of [Rh(en)3]3+, there are some intriguing resonance effects the origins of which could not be fully clarified in terms of group coupling effects. Therefore, simple and general rules that predict how the intensity of a specific normal mode is affected by resonance effects are difficult to devise.

Can the Resonance Raman Optical Activity Spectrum Display Sign Alternation?

The monosignate character of resonance Raman optical activity (RROA) spectra has been often taken as granted in experimental and computational approaches, on the basis of basic theoretical approximations only considering resonance with a single electronic state of the molecule and the scattering process to be governed by the Franck-Condon mechanism. We show in this letter for the first time that, by resorting to a fully quantum mechanical (QM) methodology able to take into account all terms entering the general definition of RROA, and which considers excited state interference and Herzberg-Teller effects, sign alternation and at the same time intensity enhancement in RROA spectra is obtained. Such features constitute an important milestone toward the exploration of RROA of a wide range of chiral biological molecules.

Through-space transfer of chiral information mediated by a plasmonic nanomaterial.

The ability to detect chirality gives stereochemically attuned nanosensors the potential to revolutionize the study of biomolecular processes. Such devices may structurally characterize the mechanisms of protein-ligand binding, the intermediates of amyloidogenic diseases and the effects of phosphorylation and glycosylation. We demonstrate that single nanoparticle plasmonic reporters, or nanotags, can enable a stereochemical response to be transmitted from a chiral analyte to an achiral benzotriazole dye molecule in the vicinity of a plasmon resonance from an achiral metallic nanostructure. The transfer of chirality was verified by the measurement of mirror image surface enhanced resonance Raman optical activity spectra for the two enantiomers of both ribose and tryptophan. Computational modelling confirms these observations and reveals the novel chirality transfer mechanism responsible. This is the first report of colloidal metal nanoparticles in the form of single plasmonic substrates displaying an intrinsic chiral sensitivity once attached to a chiral molecule.

Origin invariance in vibrational resonance Raman optical activity

A theoretical investigation on the origin dependence of the vibronic polarizabilities, isotropic and anisotropic rotational invariants, and scattering cross sections in Resonance Raman Optical Activity (RROA) spectroscopy is presented. Expressions showing the origin dependence of these polarizabilities were written in the resonance regime using the Franck-Condon (FC) and Herzberg-Teller (HT) approximations for the electronic transition moments. Differently from the far-from-resonance scattering regime, where the origin dependent terms cancel out when the rotational invariants are calculated, RROA spectrum can exhibit some origin dependence even for eigenfunctions of the electronic Hamiltonian. At the FC level, the RROA spectrum is completely origin invariant if the polarizabilities are calculated using a single excited state or for a set of degenerate states. Otherwise, some origin effects can be observed in the spectrum. At the HT level, RROA spectrum is origin dependent even when the polarizabilities are evaluated from a single excited state but the origin effect is expected to be small in this case. Numerical calculations performed for (S)-methyloxirane, (2R,3R)-dimethyloxirane, and (R)-4-F-2-azetidinone at both FC and HT levels using the velocity representation of the electric dipole and quadrupole transition moments confirm the predictions of the theory and show the extent of origin effects and the effectiveness of suggested ways to remove them.

Theoretical vibrational optical activity of chiral carbon nanoparticles: Fullerenes and carbon nanotubes

A theoretical study of the first‐order resonance Raman optical activity (ROA) of a single‐walled carbon nanotube (SWCNT) is presented for the first time. Tight‐binding (TB) Raman and ROA scattering tensors are combined with precise first principles harmonic vibrational modes to obtain scattering intensities. This computational protocol for Raman and ROA spectra has been tested previously for chiral fullerenes, such as C [Nagy et al., J. Chem. Phys. 140, 044112 (2014)]. In the present study, this methodology is validated against density functional theory for the alternative case of C. TB spectral shapes are found to be reliable to determine the absolute configuration of these cage like carbon structures with relatively small curvature. Utilizing the same method, Raman optical activity (ROA) intensities of the (6,5) SWCNT are calculated for three vibrational modes, radial breathing mode (RBM), G, and G. This tube exhibits approximately 5 orders of magnitude stronger ROA intensity compared to typical chiral molecules.

Bisignate resonance Raman optical activity: a pseudo breakdown of the single electronic state model of RROA?

Raman optical activity (ROA) spectra were calculated for a set of conformers of astaxanthin, which is a non‐rigid molecule exhibiting strong resonance enhancement in the visible range. Single electronic state theory of the Resonance ROA (RROA) predicts the spectrum to be monosigned. For astaxanthin, it appeared that some of the conformers exhibit different sign of the bands than the other conformers. As a result, the conformer population averaged spectrum of astaxanthin can exhibit both signs of the bands, or be monosigned depending on which conformers are dominating, that reflects a departure from the single electronic state approximation. Moreover, use of different basis sets and/or density functional theory (DFT) functionals results in different conformer populations, thus yielding again either monosigned ROA spectrum or with bands of both signs. Consequences of these two findings for the astaxanthin RROA spectrum are discussed. Copyright © 2014 John Wiley & Sons, Ltd.

Simultaneous Resonance Raman Optical Activity Involving Two Electronic States

In the present work, the first observation of strong resonance Raman optical activity (RROA) involving more that one resonant electronic state is reported. The chiral transition metal complex bis-(trifluoroacetylcamphorato) copper(II), abbreviated Cu(tfc)(2), exhibits both resonance Raman (RR) and RROA spectra with laser excitation at 532 nm. Vibrational assignments for this complex were carried out by comparing the non-RR spectra of Cu(tfc)(2) excited at 1024 nm to density functional theory (DFT) calculations. The theory of the single-electronic-state (SES) RROA is extended to the next simplest level of theory involving two resonant electronic states (TES) without interstate vibronic coupling as an aide to the interpretation of the observed TES-RROA spectra. Based on measured UV-vis electronic absorbance spectra and corresponding TD-DFT calculations, the most likely two states associated with the RROA spectra are identified.

Resonance Raman Optical Activity and Surface Enhanced Resonance Raman Optical Activity Analysis of Cytochrome c

High-resolution resonance Raman (RR) and resonance Raman optical activity (ROA) spectra of cytochrome c were obtained in order to perform full assignment of spectral features of the resonance ROA spectrum. The resonance ROA spectrum of cytochrome c revealed a distinct spectral signature pattern due to resonance enhanced skeletal porphyrin vibrations, more pronounced than any contribution from the protein backbone. Combining the intrinsic resonance properties of cytochrome c with the surface plasmon enhancement achieved with colloidal silver particles, the surface enhanced resonance Raman scattering (SERRS) and surface enhanced resonance ROA (SERROA) spectra of the protein were successfully obtained at concentrations as low as 1 microM. The assignments of spectral features were based on the information obtained from the RR and resonance ROA spectra. Excellent agreement between RR and SERRS spectra is reported, while some disparities were observed between the resonance ROA and SERROA spectra. These differences can be ascribed to perturbations of the physical properties of the protein upon adhesion to the surface of the silver colloids.

Experimental observation of resonance Raman optical activity

We provide the first experimental evidence for resonance Raman optical activity (RROA). In non-aqueous solution (+)-naproxen ((S)-6-methoxy-α-naphthalenacetic acid) and its deuterated methyl ester exhibit monosignate negative RROA, whereas the RROA spectrum of (−)-naproxen sodium is monosignate positive (aqueous solution above pH 10), for 514 nm excitation. In all cases, the RROA bands exhibit the same relative intensities as those in the parent resonance Raman (RR) spectrum, and the ratio of RROA to RR is comparable in magnitude, but opposite in sign, to the electronic circular dichroism (CD) anisotropy ratio of the lowest energy electronic state near 330 nm. These findings are in accord with the theory of RROA in the limit of strong resonance with a single electronic state.

Theory of resonance Raman optical activity: the single electronic state limit

The theory of natural Raman optical activity (ROA) is extended to the limit of strong resonance with a single electronic state (SES). It is shown that in the SES limit, the resonance ROA (RROA) bands all have the same sign, and their relative intensities are identical to the corresponding resonance Raman (RR) parent bands. The sign of the RROA bands are predicted to be opposite that of the pure electronic circular dichroism (CD) of the single, resonant electronic transition. The ratio of the RROA to the RR spectrum is the same or one-half the magnitude of the electronic CD anisotropy ratio of the resonant electronic state. Deviation from this simple pattern is indicative of contributions from more than one excited electronic state to the RR and RROA spectra, and hence is a sensitive probe of the electronic origin of ordinary RR spectra.

Magnetic resonance‐Raman optical activity of ferrocytochrome c theory and experiment

AbstractThe theory of magnetic resonance Raman optical activity in metal porphyrins is developed. Irreducible tensor methods are used to calculate the transition polarizabilities (involving both symmetric and antisymmetric contributions) which are formulated in terms of a vibronic development of the Raman transition tensors. The circular intensity difference for the vibrations of symmetry species A1, A2, B1 and B2 is explicitly evaluated for excitation within the 0–0 and 0–1 visible absorption bands. Polarized, depolarized and unpolarized magnetic resonance Raman optical activity spectra of ferrocytochrome c within the 0–0 and 0–1 bands are presented. Good qualitative agreement is found between theory and experiment.